This invention relates to a refining process of a hot gas containing hydrogen sulfide and ammonia, particularly to a process for removing hydrogen sulfide and ammonia from a coal gas.
The art of gasifying fossil fuel to synthesize a gaseous fuel such as methane, etc. is well known.
Recently coal has been acknowledged again as an energy resource instead of oil. In a power generation system utilizing the coal, it is necessary from the standpoint of resource saving and energy saving to combust the synthesized gaseous fuel of elevated temperature without lowering its temperature to operate a gas turbine. However, the fossil fuel such as coal inevitably contains sulfur and nitrogen which are converted to hydrogen sulfide and ammonia, respectively, when gasified. The synthesized gaseous fuel contains the hydrogen sulfide and ammonia at concentration from 1000 ppm to several percentages. As the hydrogen sulfide is a highly corrosive gas, and the ammonia is converted to nitrogen oxide when the synthesized gaseous fuel is combusted, both of them can not be discharged into the atmosphere on the ground of environmental pollution. Further from the ground of protection of the gas turbine and economy, also, it is necessary to remove the hydrogen sulfide and ammonia from the synthesized gaseous fuel before the combustion. Therefore, development of the removing art of the hydrogen sulfide and ammonia at an elevated temperature is demanded as a key point for the success of the power generation system using coal as a fuel resource.
Hitherto, as a method of removing hydrogen sulfide in a hot gas such as the coal gas at a high temperature, a dry method using a granular solid removing agent is known to be effective. As there moving agent, calcium carbonate dolomite, iron oxide etc. are known. The iron oxide among these agents is said to be most excellent from the standpoints of a removal ratio of the hydrogen sulfide, easiness of regenerating the removing agent weakened in its activity, economy etc.
The iron oxide such as ferric oxide reacts with the hydrogen sulfide to form Fe S according to the following equation (1). EQU Fe.sub.2 O.sub.3 +2H.sub.2 S+H.sub.2 .fwdarw.2FeS+3H.sub.2 O (1)
After the ferric oxide is converted to the iron sulfide, the ferric oxide loses the ability of the removal of the hydrogen sulfide, so that it is necessary to restore the removal ability by regenerating. As a method of regenerating the ferric oxide, a method is used such that the iron sulfide is contacted with air, or steam mixed with air to restore it to the iron oxide while producing SO.sub.2 gas or H.sub.2 S, which reactions are as follows: EQU 4FeS+7O.sub.2 .fwdarw.2Fe.sub.2 O.sub.3 +4SO.sub.2 ( 2) EQU 2FeS+3H.sub.2 O.fwdarw.Fe.sub.2 O.sub.3 +2H.sub.2 S+H.sub.2 ( 3)
Thus, since when the H.sub.2 S is removed the production of the FeS accompanies the production of water, water component in the gas restrains the H.sub.2 S removal reaction shown by the equation (1). Therefore, a concentration of the H.sub.2 S determined by both of a gas temperature and a water concentration of the gas remains.
It is known by U.S. patent application Ser. No. 771,911 filed Feb. 25, 1977 that ammonia is decomposed by the iron oxide. However the coal gas including both the ammonia and hydrogen sulfide is not decomposed by the iron oxide at a high decomposition rate. This is because the hydrogen sulfide and water work to obstruct the ammonia decomposition, which is explained later in detail. It is difficult to remove completely the hydrogen sulfide in its removal process, a certain concentration of the hydrogen sulfide remains as abovementioned. The water is included in the coal gas, further it is formed also through the reaction (1).
Accordingly the ammonia decomposition is not effective enough.
The other relevant prior arts are as follows
1. U.S. Pat. No. 3,812,236 PA0 2. U.S. Pat. No. 3,822,337
"Removal Of Ammonia And Organic Impurities From An Ammonia Plant". PA1 "Process For Elimination Of Ammonia And Hydrogen Sulfide From Gases Generated in Coke Plants And Gas Works".